Fix nullptr passed to memcpy in lld/COFF/Chunks.cpp

Summary:
ubsan found that we sometimes pass nullptr to memcpy in
SectionChunk::writeTo(). This change adds a check that avoids that.

Reviewers: ruiu

Reviewed By: ruiu

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D45789

llvm-svn: 330490